A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As known semiconductor thin films which can be used for transistors, a silicon-based semiconductor material, an oxide semiconductor, and the like are used.
In order to achieve high speed operation of a transistor, miniaturization of the transistor is needed. For example, in Patent Document 1, a thin film transistor including an oxide semiconductor used for a channel layer with a thickness of about 10 nm or smaller is disclosed. In Non-Patent Document 1, a thin film transistor including an oxide semiconductor whose channel length is 2 μm to 100 μm is disclosed.